Due to the increasing inlet gas temperature of gas turbines, there is a need to ensure the reliability and durability of, for example, a section near the main flow and the inside of an axial cavity (i.e., a gap in the axial direction) formed between a rotor-blade rotating disk and a stator-blade ring at the upstream side or the downstream side thereof with a smaller amount of sealing air (i.e., air leaking from the inside of the cavity toward the main flow and having a temperature lower than that of the main flow).
Generally, in order to improve the reliability of axial cavity sections between blade rows, it is necessary to increase the amount of sealing air in the axial cavities to prevent the entry of high-temperature main-flow gas. However, increasing the amount of sealing air leads to reduced performance of the gas turbine.
Furthermore, although the sealing air leaking into the main flow from the axial cavities between the blade rows is a gas with a temperature lower than that of the main flow and has an ability to cool the blade surfaces (including a shroud and a platform surface), since a flow pattern produced by the blades is dominant in the main-flow section, such as the blade surfaces, it is difficult to effectively perform cooling using this leaking sealing air.
It is known from the operation histories and rig tests of gas turbines, as well as from NPL 1, that the pressure distribution and temperature distribution as well as vortexes at several large circumferential intervals around the entire circumference (360 degrees) or at large intervals over a circumferential range covering multiple blades tend to occur inside the axial cavities due to the nature of the flow, in addition to the pressure distribution and gas temperature distribution occurring at small circumferential intervals every other stator blade or rotor blade in the main flow. Furthermore, in an actual gas turbine engine, it is structurally difficult to achieve perfect symmetry in the circumferential direction, and structural asymmetry in the circumferential direction is one factor that causes a pressure distribution of one to several cycles to occur over the entire circumference within the axial cavities.
Therefore, during the design process, it is necessary to set an extra amount of sealing air in view of such unevenness in the circumferential direction.
On the other hand, it is known from a Computational Fluid Dynamics (CFD) analysis and from NPL 2 that the pressure distribution and temperature distribution around the entire circumference occur not only at the cavity side, but also at the main-flow side depending on a blade-count difference between stator blades or a blade-count difference between rotor blades, as shown in, for example, FIG. 9. The problem in this case is the occurrence of the distribution at large intervals of one to four cycles over the entire circumference, which can occur when the blade-count difference is small, such as about one to four. Regarding such a distribution over a circumferential range covering multiple blades, it is difficult to achieve uniformity, compared with the pressure distribution and temperature distribution occurring every other blade.
In order for the sealing air to leak into the main flow from the axial cavities between the blade rows, the pressure within the cavities needs to be higher than that of the main flow. However, in the case where the pressure distribution at large intervals (in small cycles between about one and four) occurs at both the axial-cavity side and the main-flow side, as described above, when a low in the pressure distribution on the axial-cavity side and a high in the pressure distribution on the main-flow side are aligned with each other, there is an increased risk of main-flow gas over the circumferential range covering multiple blades entering the axial cavities between the blade rows. With such a configuration, the durability of the components is significantly reduced.
PTL 1 discloses changing the relative position, in the circumferential direction, between stator blades in front and rear stages (that is, clocking) so as to make a wake flow produced by a blade surface of the upstream stator blade reach the downstream stator blade, thereby improving the performance.
PTL 2 discloses a method that changes the relative position, in the circumferential direction, between the stator blades in front and rear stages so as to cool the downstream blade using a wake flow produced by the upstream stator blade or using cooling air blowing out from the upstream stator blade.